Visitors are redirected only during first visit/ visitors are not able to switch store view in magento.

You have two patterns of users’ redirection:

1) redirect customers only on their first visit. In this case they will be able to switch between available store views and currencies manually;

2) always use the redirection rules, so your clients won’t be able to choose other languages/currencies.

This setting can be controlled in System > Configuration > Restriction > First visit redirect only set: